1. Field of the Invention
The present invention relates to a photodetector device, a solid-state imaging device comprising a plurality of the photodetector devices, and a camera system comprising the solid-state imaging device.
2. Description of the Related Art
Semiconductor image sensors (e.g., CCD and MOS image sensors, etc.) have been applied to a number of image input devices. Particularly, in recent years, MOS image sensors have been reviewed and have attracted attention because their power consumption is low and they can be produced using the same CMOS technology as that which is used for their peripheral circuitry. In the field of MOS image sensors, attention has been paid to a so-called logarithm conversion image sensor. The logarithm conversion image sensor utilizes a subthreshold region of a MOS transistor, has logarithmic characteristics of the amount of irradiation and the output, and operates in a high dynamic range (see, for example, Japanese Laid-Open Publication No. 2003-158683).
FIG. 10 shows a photodetector device 900 which is provided in each unit pixel of a logarithm conversion image sensor. In the photodetector device 900, an output voltage Vout is determined so that a photocurrent Ipd generated by a photodiode 901 is balanced with a subthreshold current Ids of a logarithmic conversion MOS transistor 902.
The subthreshold current Ids of the MOS transistor 902 is represented by:Ids=Io×exp((φg−Vs)q/kT)  (1)where Io is a proportionality factor, q is the charge per electron, k is the Boltzmann constant, T is an absolute temperature, φg is a surface potential of a lower portion of the gate, and Vs is a potential of the source.
The relationship between a gate voltage Vg and the surface potential φg is represented by:Vg=mφg+a  (2)where m=dVg/dφg=1+Cd/Cox≈1.1 (Cox: oxide capacitance, Cd: capacitance of a depletion layer below the gate), and a is a constant. According to expressions (1) and (2), expression (3) is obtained:Ids=Io×exp(((Vg−a)/m−Vs)q/kT)  (3).
Assuming that Ids=Ipd and Vg=Vdd according to FIG. 10, Vout (=Vs) is represented by:Vout=(Vdd−a)/m−(kT/q)ln(Ipd/Io)  (4).
As can be seen, the output Vout and the photocurrent Ipd have logarithmic characteristics. When different amounts of light generate photocurrents Ipd1 and Ipd2 and the resulting output voltages are Vout1 and Vout2, the following expression is obtained:Vout2−Vout1=−(kT/q)ln(Ipd2/Ipd1) (5).
For example, when it is assumed that the amount of light is increased by a factor of 10 at room temperature (i.e., Ipd2=Ipd1×10), the following expression is obtained:Vout2−Vout1=−(kT/q)ln(10)≈−60 mV  (6).
This expression indicates that the amount of change in the output voltage with respect to the rate of change in the amount of light depends only on the absolute temperature, i.e., has a constant value. When the rate of change in the amount of light is assumed to be α, the amount of change in the output voltage is represented by:ΔVout=−(kT/q)ln(α)  (7).
Note that an image sensor is composed of a plurality of photodetector devices arranged in a two-dimensional manner, each photodetector device may have a structure as shown in FIG. 11 (photodetector device 910). The photodetector device 910 comprises a pixel amplifier 911 for impedance conversion and a row selection switch portion 912 in addition to the components of the photodetector device 900. A signal is generally output on a row-by-row basis. In the photodetector device 910, when a voltage is applied to a signal line SEL, the row selection switch portion 912 is switched ON, so that a signal output from the pixel amplifier 911 is output onto a column output line 913.
MOS transistor characteristics vary between each logarithmic conversion MOS transistor 902, which operates in a subthreshold region. Therefore, it is known that when an operating point of the logarithmic conversion MOS transistor 902 varies between each pixel, so-called two-dimensional fixed pattern noise occurs. To reduce the fixed pattern noise, a circuit for outputting a difference between an output signal and a reset signal in each pixel (CDS circuit) is provided (see, for example, Japanese Laid-Open Publication No. 2001-245214).
FIG. 12 shows a photodetector device 920. The photodetector device 920 comprises the components of the photodetector device 910, and in addition, a constant current power source 914, and a switch portion 915, which is connected to the constant current power source 914 and the logarithmic conversion MOS transistor 902.
FIG. 13 shows an example of driving pulse timing of the photodetector device 920. At output signal read time t1, a SEL switch is switched into conduction to output an output signal onto a column output line. In this case, the waveform of Vout has a high potential in the dark and a low potential in the light.
Next, at time t2, the switch portion 915 is switched into conduction so that the constant current power source 914 generating a constant current which is much larger than the output signal is brought into conduction with the source electrode of the logarithmic conversion MOS transistor 902. As a result, a source follower circuit whose input is Vdd is formed. This situation is equivalent to when an excessive amount of light is incident to the photodiode 901, and the potential of Vout is sufficiently lowered no matter whether it is in the dark or light.
Thereafter, at time t3, the SEL switch is switched into conduction, and when the potential of Vout is sufficiently lowered, an output signal is output as a reset signal to the column output line 913. The reset signal contains a variation in characteristics of the logarithmic conversion MOS transistor 902 (more specifically, also a variation in characteristics of the pixel amplifier 911) Therefore, a subsequent CDS circuit (not shown) is used to cancel the characteristics variation of the logarithmic conversion MOS transistor 902 in each pixel, resulting in a reduction in the fixed pattern noise. At time t4 and thereafter, the switch portion 915 is switched OFF, and an ordinary photodetection is continued until the next read operation (after one frame period).
In the above-described conventional photodetector device, when the rate of change in the amount of light is 10-fold, the amount of change in the output voltage is about 60 mV according to expression (6). An upper limit of an incident light amount range in which a photodetector device can operate is an amount of light (high luminance) which generates a large current which causes the operation of a logarithmic conversion MOS transistor to deviate from the subthreshold region. A lower limit of the incident light amount range is an amount of light (low luminance) which generates a small current in which a dark current component generated in the photodiode is not negligible. The light amount range is about 160 dB(=108). Therefore, the amount of change in the output voltage is only about 60 mV×8=480 mV. Compared to conventional linear conversion image sensor in which the amount of change in the output voltage is about 1 V, the logarithm conversion image sensor has an insufficient dynamic range of the output signal. In the case of the logarithm conversion image sensor, a gain is inevitably increased using an analog amplifier in order to A/D convert an output signal efficiently, leading to a large noise and thus a degradation in image quality. As described above, the logarithm conversion image sensor disadvantageously has a small absolute amount of change in the output voltage compared to conventional linear conversion image sensors.